Conventionally, enclosure and protection of contents during storage, transportation or the like by encapsulating the contents tightly with a packaging material in the form of a tube or a flat film by heat sealing (lock seal) has been carried out. As for the improvement of seal strength in forming a lock seal, various investigations on aspects such as packaging materials, packaging machines, packaging conditions and the like have been conducted, and thus, for example, as the packaging materials, ethylene-based polymers such as polyethylene and an ethylene-vinyl acetate copolymer are known. Among these, ionomers are widely used since they are excellent in, for example, hot tack property, low-temperature seal property, and oil resistance.
On the other hand, when it is desired that the contents be taken out of the packaging material while keeping the contents intact, with a certain degree of a lock seal property, a method for heat sealing in which a sealing section can be peeled and opened by an opening force (peelable seal) is also carried out.
For example, a polymer material containing 80 to 93% by mass of an ethylene/acid ionomer and 7 to 20% by mass of a propylene/α-olefin copolymer is disclosed (see, for example, Patent Document 1), in which the lock seal and the peelable seal can be formed under the control of temperatures. When this polymer material is used to form a film or a laminate for filling and packaging by a packaging machine, it is possible to obtain a package having both a lock seal section and a peelable seal section in one packaging material, by forming the lock seal on the back side or bottom under high temperature conditions and forming the peelable seal at the top under low temperature conditions. As the polymer material to be used for such a package, for example, a laminated packaging material in which a base layer such as a polyester layer, and an ionomer layer are laminated with an adhesive layer interposed therebetween, and the like are known. In this laminated packaging material, it is necessary to exhibit a seal strength of about 4 N/10 mm width under low-temperature sealing conditions (e.g.: about 100 to 130° C.) and a seal strength of 10 N/10 mm width or higher at high-temperature sealing conditions (e.g.: 150° C. or higher) in terms of the relationship between the sealing temperature and the seal strength.
Furthermore, in practice, it is necessary to stably form the lock seal and the peelable seal. In particular, it is required to have a temperature range in which the peelable seal can be formed stably, that is, the temperature range in which the seal strength of the peelable seal section does not vary significantly depending on the temperature.
As a technique regarding such a seal form, a laminated film for packaging, which is provided a seal layer of a polymer composition formed of 60 to 95 parts by mass of a metal salt of an ethylene/α,β-unsaturated carboxylic acid-based copolymer and 40 to 5 parts by mass of an ethylene/α,β-unsaturated carboxylic acid ester copolymer, is disclosed (see, for example, Patent Document 2). Here, it is considered that a seal strength that is relatively low and also constant over a wide temperature range can be obtained, and a high seal strength is exhibited under high-temperature sealing conditions.
Furthermore, a blend of an ionomer containing 5 to 25% by mass of a unit derived from isobutyl acrylate, in addition to a unit derived from ethylene and an acid, and a propylene copolymer is disclosed (see, for example, Patent Document 1).
Moreover, as for a means for establishing a difference in the seal strength by changing the seal temperature, it is disclosed that a sealant layer of a composition containing an ionomer resin or a composition obtained by the blending of two or more kinds of resins having different melting points and an ionomer resin is provided as the seal layer (see, for example, Patent Document 3). The sealant layer including this ionomer resin is characterized in that there is a slope rising gently toward one side along with the elevation of the seal temperature.
In addition, there is a disclosure of a seal layer having an easily peelable property, in which the seal layer is formed of the composition of an ionomer, an ethylene/methacrylic acid copolymer, polypropylene, and an inorganic filler (see, for example, Patent Document 4). The seal layer including this composition is considered to exhibit a seal strength of 1.5 to 2 N/15 mm.